This invention relates to electronic triggers for Radio Frequency (RF) sources and more particularly to a highly stabilized electronic trigger for an RF source such as is used in a radar transmitter.
Electromagnetic range finding systems, such as radars, frequently emit an RF signal for the purpose of detecting targets which reflect the transmitted RF signal back to the range finding apparatus. Of course, as is well known, the time difference between the time the RF signal is emitted and the time at which it is received is indicative of the distance the target is from the transmitter and a changing phase of the returned signal is indicative of a moving target. Radar systems capable of detecting this phase change are known as Moving Target Indication (MTI) radars.
It is of considerable importance to designers of such apparatus that not only the time at which the RF signal is transmitted be known but also that the RF signal be transmitted at a preselected time or at least within a preselected period. The transmitted RF signal originates, of course, at an RF source which may be provided, for example, by a magnetron, a klystron, or a more conventional oscillator coupled to an RF power tube. The magnetron is a popular RF source for use with radar transmitters because of its compact size, low weight, low cost, high efficiency and low source of X-rays. However, the magnetron also has several disadvantages which include the fact that it has no phase coherence between consecutive pulses and because it is subject to drift with respect to the time at which the RF signal leaves the magnetron as compared to the time at which it is triggered. This lack of coherence, as will be realized by those skilled in the art, would effectively proscribe use of magnetrons in Moving Target Indication (MTI) radars unless compensatory measures were taken. The coherence problem is overcome, as is well known, by taking a sample of the transmitted pulse at a directional coupler, mixing this pulse with the output of a stabilized local oscillator and using the resulting pulse to phase lock a coherent oscillator. The coherent oscillator then becomes a reference oscillator against which the received signals are compared for phase variation. However, the output of the magnetron, the magnetron current pulse (MCP), is typically delayed on the order of 7.5 microseconds from its trigger and may drift on the order of .+-.1.0 microseconds or more, due to variations in temperature, line voltage and so forth. As will be appreciated by those skilled in the art, this drift can degrade the overall MTI performance of a modern radar system.
It was, therefore, one object of this invention to reduce the effects of drift on performance of an MTI radar.
It is yet another object of this invention that an electronic trigger be provided for a magnetron whereby the magnetron current pulse emitted thereby will be emitted at a preselected time with a high degree of accuracy.
It is yet another object of this invention to improve electronic triggers for RF sources.
It was still another object of this invention that the electronic trigger be inherently stable.
The foregoing objects are achieved as is now described. A counter is loaded with a selected multi-bit binary number. This number is preferably derived from the contents of a delay accumulator. As will be seen, the magnitude of the number in the delay accumulator is adjusted up or down depending upon whether or not the RF energy is generated at the correct time. Receipt of a pretrigger signal causes the counter to initiate counting and upon reaching a predetermined value it generates a modulator trigger. The modulator trigger modulates the RF source, such as a magnetron, and sometime thereafter the RF source generates its RF energy. In the magnetron embodiment, the occurrence of the generated Magnetron Current Pulse (MCP) is compared with a timing signal which is preferably generated at the time that the MCP should desirously appear. Of course, in actual operation, the MCP will typically lead or lag this timing signal. A phase detector samples both the MCP and this timing signal and adjusts the magnitude of the number in the delay accumulator when the MCP leads or lags the timing signal by a predetermined amount (about 50 nanoseconds in the disclosed embodiment). Typically, the number in the delay accumulator is randomly selected when the system is first energized, decremented thereafter for each cycle in which the MCP leads the timing signal by at least the predetermined amount or incremented thereafter for for each cycle in which the MCP lags the timing signal by at least the predetermined amount.
The advantage of the invention, both as to its construction and mode of operation, and the preferred mode of use, it will be readily appreciated by those skilled in the art referring to the following detailed description of an illustrative embodiment when considered in conjunction with the accompanying drawings in which like reference numerals refer to the same composite throughout the drawings.